White Envelope Icon Showing in Status Bar

That should be an email notification - what does it say when you drag down the notification bar?

 

^^^ Either that or a voice mail. The voice mail looks like a white envelope with a "tape" icon in the center. Either way, drag it down and see what it says.

 

You are probably getting alerted twice. While using handcent you need to turn off the alerts in the native messaging app.

 

^^ +1